1. Field of the Invention
The invention relates to a process for producing high performance fibers and nonwoven insulating webs including such fibers, which webs are particularly suited for use as garment or sleeping bag interlinings. More specifically, the invention concerns an insulating web which includes a mass of metal coated glass or synthetic polymer fibers, and to a process for producing same.
2. Description of the Prior Art
The commonly practived technology for producing insulation webs is to fashion webs composed of a mass of fine fibers. The fibers stop any gaseous convection and somewhat block radiation heat transfer by causing a multitude of fiber to fiber radiation exchanges. In each exchange, some radiant energy is blocked from moving through the pack. If one wants to further reduce the radiation heat transfer, more fibers are added.
Many nonwoven materials have been suggested and used for insulating interliners. J. L. Cooper and M. J. Frankosky, "Thermal Performance of Sleeping Bags" Journal of Coated Fabrics, Volume 10, pages 108-114 (October 1980 compares the insulating value of various types of fibrous materials that have been used as interliners in sleeping bags and other articles. Among the products compared are polyester fiberfill of solid or hollow or other special fibers and a product of 3M Company (St. Paul, Minn.) called Thinsulate.RTM.. Generally, polyester fiberfill is made from crimped polyester staple fiber and is used in the form of quilted batts. Usually, batt bulk and bulk durability are maximized in order to increase the amount of thermal insulation. Hollow polyester fibers have found widespread use in such fiberfill batts because of the increased bulk they offer, as compared to solid fibers. In certain fiberfill materials such as Hollowfil.RTM.II, a product of E. I. du Pont de Nemours and Company (Wilmington, Del.), the polyester fibers are coated with a wash-resistant silicone slickener to provide additional bulk stability and fluffability. For fiber processability and in-use bulk, slickened and non-slickened fiberfill fibers for use in garments have usually been in the range of 5 to 6 denier (22 to 25 microns diameter). A special fiberfill, made from a blend of slickened and non-slickened 1.5 denier polyester staple fibers and crimped polyester staple fiber having a melting point below that of the other polyester fibers, in the form of a needle-punched, heat-bonded batt, is reported to exhibit excellent thermal insulation and tactile aesthetic properties. Such fiberfill batts are also discussed in U.S. Pat. No. 4,304,817. "Thinsulate" is an insulating material in the form of a thin, relatively dense, batt of polyolefin microfibers, or of the microfibers in mixture with high denier polyester fibers. The high denier polyester fibers are present in the "Thinsulate" bats to increase the low bulk and bulk recovery provided to the batt by the microfibers alone. For use in winter sports outerwear garments, these various insulating materials are often combined with a layer of film of porous poly (-tetrafluoroethylene) polymer of the type disclosed in U.S. Pat. No. 4,187,390.
Although the above-described prior art nonwovens have been useful as insulating interliners, various improvements would significantly enhance their utility. For example, it has been known for many years that if the optical properties of the fibers are changed, the radiation heat transfer can be changed. The reference "Thermal Insulation: What It Is and How It Works" by Charl M. Pelanne in the Journal of Thermal Insulation, Vol. 1 (April 1978) teaches that radiation can be controlled by the emittances of the surfaces involved or by the insertion of absorbing or reflecting surfaces (sheet, fibers, particles, etc.) between the two temperature boundaries. The article "Analytical Models For Thermal Radiation In Fibrous Insulations" by T. W. Tong and C. L. Tien in the Journal of Thermal Insulation, Vol. 4 (July 1980) attempts to quantify the effect by creating models for heat transfer in fibrous insulations.
Now, even though it has been known for many years that modifying the optical properties of the fibers can be beneficial, the difficulty has been in establishing a commercially acceptable process of modification. These properties can be modified some by changing the composition of the fibers but not to the extent necessary to obtain the lowest heat transfer.
What is desired is a fiber that neither absorbs nor radiates radiant energy. This would be a fiber with an emissivity of 0 and an absorbtivity of 0. Some materials are known to have very low emissivities and absorbtivities such as gold (0.02), silver (0.02), and aluminum (0.04). Fibers made of these materials could be produced but they would be expensive, heavy, exhibit plastic deformation instead of elastic deformation, and exhibit other limiting properties.
What would be clearly desirable is to coat fibers made out of the desired fiber material with a material which would modify the surface of the fiber to yield a low emissivity/absorbtivity.
Since most of the fibers of interest, such as polymers and glass, are nonconductive, electroplating is not possible. Electroless plating is possible but many of the materials that can produce a low emissivity can not be used as coating materials by this method. Aluminum is an example.
One method which would be highly desirable would be to vacuum metallize the fibers. Unfortunately, this method can only coat in a straight line of a sight. Fibrous insulating webs are comprised of so many fibers that a straight line of sight coating would coat less the 7 percent of the fibers in a typical web that is 0.5 inch thick and 0.5 pounds per cubic foot density.
The process taught by Foragres, Melamed, and Welner in U.S. Pat. No. 4,042,737 is well suited for wet processing where continuous metal plated filament or yarn is required, but has major deficiencies where metal coated staple fiber is desired. The knitting process is very slow (approximately 100 grams of 40 microns continuous nylon fiber per hour) and becomes much slower and more difficult when the fiber denier is in the desired range for thermal insulation (less than about 25 microns). If a continuous yarn is used instead of a filament in order to increase through-put, the internal filaments of the yarn would not be metal coated in a vacuum metallization process.
Thus the problem: for years scientists have known that a low emissivity coating on fibers used in insulation webs would be desirable. However, there has been no practical method for producing the coated fibers for use in the webs.